Stabilization of amine-containing CO2 adsorbents and related systems and methods

ABSTRACT

The present invention provides (i) a method and system for stabilizing the performance of amine-containing CO2 adsorbents using wet feed gas and/or wet purge gas and (ii) a method for regeneration of deactivated amine-containing CO2 adsorbents via hydrolysis of the urea groups formed during deactivation.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No. 12/976,557, filed Dec. 22, 2010, and claims priority to and the benefit of U.S. Provisional Patent Application No. 61/289,198, filed Dec. 22, 2009, entitled “Stabilized Amine-Containing CO₂ Adsorbents and Related systems and Methods,” which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention pertains to the field of adsorption methods and systems for carbon dioxide capture, more particularly, to the field of (i) adsorption methods and systems, (ii) stabilization methodologies of amine-containing adsorbents for CO₂ separation and the purification of carbon dioxide-containing gases, and (iii) regeneration of deactivated amine-containing CO₂ adsorbents

BACKGROUND

Capture of anthropogenic carbon dioxide from large sources of emission such as fossil fuel power plants is a key target in the ongoing effort to mitigate the effect of greenhouse gases on global climate change. Although mature, the liquid phase amine scrubbing technology suffers from inherently high regeneration cost, equipment corrosion and amine oxidative degradation. As a result, there is a strong tendency to develop recyclable solid sorbents to achieve competitive, less energy intensive acid gas removal alternatives.

Most physical CO₂ adsorbents such as 13× zeolite, activated carbons, periodic mesoporous silica and metal-organic frameworks (MOFs) require a large pressure and/or temperature gradient between the adsorption and desorption stages to enable both efficient adsorption performances and near complete desorption of CO₂. Moreover, they exhibit relatively low selectivity toward CO₂, generally low tolerance to water vapour in the gas feed, and their CO₂ separation performance decreases drastically by increasing the temperature.

Incorporation of amine groups onto large surface area porous solids via direct synthesis, impregnation, surface polymerization, surface grafting or co-condensation as a promising approach for CO₂ capture has gained prominence in recent years. When properly designed, such materials exhibit high adsorption capacity, fast CO₂ adsorption and desorption and low energy recycling requirement.

In addition to the above-mentioned attributes, amine-containing materials are tolerant to the occurrence of moisture in the feed. Actually, CO₂ adsorption is enhanced by the presence of moisture in the feed (Serna-Guerrero et al. 2008). The reason lies within the nature of the amine-CO₂ interactions. Under dry adsorption conditions, surface amine groups interact with CO₂ to form carbamate with a stoichiometric CO₂/N ratio of 0.5, whereas under proper humidity conditions, bicarbonate with a stoichiometric ratio CO₂/N=1 may be formed (Serna-Guerrero et al. 2008). In contrast, in the presence of other adsorbents such as zeolites and activated carbons, CO₂ adsorption is inhibited by moisture because of unfavourable competition.

However, despite the large number of contributions devoted to CO₂ adsorption over amine-containing materials (Choi et al. 2009), and despite the utmost importance of the long term stability of such materials, no studies addressed the issue of adsorbent stability through extensive recycling.

The lifetime of adsorbents, which determines the frequency of their replacement, is a critical parameter of equal importance as CO₂ adsorption capacity, selectivity and kinetics, having a direct impact on the economics of any commercial scale operation.

This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.

SUMMARY

An object of the present invention is to provide a method and a system for stabilizing amine-containing adsorbents, for example, for use in steady state operation of amine-containing adsorbents for the purpose of CO₂ capture and removal.

In accordance with one aspect, there is provided a method of stabilizing an amine-containing CO₂ adsorbent comprising contacting said amine-containing CO₂ adsorbent with one or more humid gas streams. This method can be employed in a process for selectively removing or recovering CO₂ from a gaseous stream or atmosphere containing CO₂.

It has now been found that the stability of amine-containing CO₂ adsorbents used in a system for removal of CO₂ is enhanced when adsorption and desorption stages are carried out in the presence of moisture-containing feed and purge gases. Furthermore, the CO₂ adsorption capacity of triamine-grafted mesoporous silica demonstrates a gradual decrease upon adsorption-desorption cycling under mild conditions. In contrast, the stability of triamine-grafted mesoporous silica drastically improves in the presence of moisture-containing gases. The loss in stability of amine-containing adsorbent material has been found to be linked to the formation and accumulation of urea groups even under mild, but dry conditions, while the formation of urea group is inhibited in the presence of moisture.

In accordance with another aspect, there is provided a method for regeneration of a deactivated amine-containing CO₂ adsorbent comprising hydrolyzing urea groups in said deactivated adsorbent at high temperature in the presence of moisture.

In accordance with another aspect, there is provided a system for removal of CO₂ from a gaseous stream comprising: (a) one or more sorbent beds comprising an amine-containing adsorbent; (b) means for controlling a CO₂-containing feed gas flow through said one or more sorbent beds; (c) means for controlling a purge gas flow through said one or more sorbent beds; (d) means for controlling the relative humidity of said feed gas, said purge gas or both said feed gas and said purge gas. The system for CO₂ removal can include means of removing the CO₂ from the sorbent bed by pressure-swing (PS), vacuum-swing (VS), temperature-swing (TS) regeneration modes or a combination thereof in fixed, moving or fluidized beds; a means of adjusting the relative humidity of the gaseous stream used for adsorption and/or desorption, and a means of adjusting the level of hydration of the sorbent bed during adsorption and/or desorption.

In accordance with another aspect, there is provided a system for removing or recovering CO₂ from an gaseous stream or atmosphere containing said CO₂ using amine-containing CO₂ adsorbents with enhanced stability in terms of CO₂ capture performance over hundreds of adsorption-desorption cycles in the presence of moisture-containing feed gas and/or moisture-containing purge gas in the range of about 0.1% to 100% relative humidity (RH).

In accordance with another aspect of the invention, there is provided a system for removing or recovering CO₂ from a gaseous stream or atmosphere containing said CO₂ using different selective amine-containing CO₂ adsorbent where the purge gas may be moisture-containing non-adsorbing gas, moisture-containing CO₂ or steam.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 graphically depicts dynamic CO₂ adsorption over a number of CO₂ adsorption-desorption cycles on mono amine and triamine grafted pore-expanded MCM-41 (MONO and TRI, respectively) and polyethyleneimine-impregnated pore-expanded MCM-41 (PEI) carried out under various conditions using dry feed and purge gases.

FIG. 2 graphically depicts dynamic CO₂ adsorption over a number of CO₂ adsorption-desorption cycles on mono amine and triamine grafted pore-expanded MCM-41 (MONO and TRI, respectively) and polyethyleneimine-impregnated pore-expanded MCM-41 (PEI) carried out under various conditions using wet feed and purge gases.

FIG. 3 depicts ¹³C CP MAS NMR for a series of MONO samples and UREA (bis(trimethoxysilylpropyl)urea-grafted MCM-41).

FIG. 4 depicts in situ DRIFT spectra for fresh MONO (MONO-Fresh), MONO after 40 cycles under dry (MONO-105/105-d) and humid (MONO-105/105-h) conditions and for fresh UREA (UREA-Fresh).

FIG. 5 depicts CO₂ adsorption-desorption cycles on TRI-PE-MCM-41 in dry and humid (20° C. as dew point, 7.5% RH) conditions using adsorption and desorption at 70° C.

FIG. 6 depicts ¹³C NMR spectra for fresh MONO (MONO-Fresh), MONO after cycling in dry (MONO-105/105-d) condition, and MONO-105/105-d after hydrolysis (MONO-105/105-d-hydr).

FIG. 7 depicts ¹³C NMR spectra for fresh UREA (UREA-Fresh), UREA-Fresh after hydrolysis (UREA-Fresh-hydr) and MONO-Fresh.

FIG. 8 depicts DRIFT spectra for fresh UREA (UREA-Fresh), UREA-Fresh after hydrolysis (UREA-Fresh-hydr) and MONO-Fresh.

DETAILED DESCRIPTION OF THE INVENTION

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

As used in the specification and claims, the singular forms “a”, “an” and “the” include plural references unless the context clearly dictates otherwise.

The term “comprising” as used herein will be understood to mean that the list following is non-exhaustive and may or may not include any other additional suitable items, for example one or more further feature(s), component(s) and/or ingredient(s) as appropriate.

It has now been shown that during CO₂ adsorption-desorption cycling under dry conditions, amine-containing adsorbent materials ultimately deactivate, even under mild conditions, through formation of urea groups on and/or in the adsorbent materials. In order to minimize deactivation, the present invention provides a method to enhance the stability of such amine-containing materials. The present invention further provides a method for regeneration of the deactivated adsorbent materials.

The present application provides a method of stabilizing amine-containing materials used as CO₂ adsorbents. The amine-containing adsorbents retain a stable adsorption capacity when used under humid conditions. Accordingly, the present invention provides a method of stabilizing an amine-containing CO₂ adsorbent that includes the step of contacting the adsorbent with a gas containing CO₂ and possibly other acid gases, having a relative humidity of at least about 0.1%, and/or the step of regenerating the adsorbent in the presence of a purge gas with a relative humidity from about 0.1% to about 100%.

Amine-Containing Adsorbent

The amine-containing adsorbents useful in the method and system described herein are large surface area porous solids that include amines that are accessible to the adsorbate during an adsorption process to remove CO₂ from a gas.

Generally, there are three major classes of amine-containing adsorbents suitable for CO₂ removal, namely (i) materials with surface-anchored amine species prepared by aminosilane grafting or cocondensation or in-situ surface polymerization of reactive amines such as aziridine (Hicks et al., 2008), (ii) amine-containing species such as ethanolamine (Franchi et al., 2005) or polyethyleneimine (Yue et al. 2008, Ma et al. 2009), impregnated onto materials with high surface area such as carbon or mesoporous silica, and (iii) amine-containing MOFs (Couck et al. 2009; Vaidhyanathan et al. 2009; Arstad et al. 2008). A suitable adsorbent can be prepared using various methods, including those already known, in order to obtain CO₂ adsorbents having varying capacities and rates of adsorption depending on the potential use of the material.

Non-limiting examples of suitable amine-containing adsorbents are monoamine and triamine-grafted pore-expanded MCM-41 mesoporous silica, polyethyleneimine-impregnated PE-MCM-41, and amine-functionalized MOFs.

The amines used in the preparation of the adsorbent must exhibit sufficient basicity to allow for efficient reaction with CO₂, and/or other acidic gases to be adsorbed. In addition a high N/C ratio can be beneficial to maximising the concentration of amine groups within the adsorbent. In order to allow effective regeneration of the adsorbent, the adsorbent should be thermally stable during the desorption process.

The amines may be primary amines, secondary amines, tertiary amines, mixed amines or any combination thereof. As shown in the following section, amines can be introduced via different routes including (i) grafting or co-condensation using amine-containing trialkoxy- or trichlorosilanes, (ii) adsorption, (iv) impregnation, (iv) surface polymerization of amine-containing monomers, (v) synthesis or post-synthesis pore expansion using amines, (vi) reaction with framework or with pending reactive groups, and (vii) self assembly with silica or organosilica precursors using amphiphile amines. The means by which amines are introduced to solid supports will change depending on the type of support material used, the desired adsorbent configuration, the nature of the amine, etc.

Selection of the specific amine or amines to be used in the preparation of the adsorbent of the present invention will depend on the configuration of the adsorbent and on the application for which the adsorbent is intended. For example, in cases where a high adsorptive capacity is not critical, the amine or amines will be selected keeping in mind characteristics such as high regeneration ability, low cost and ready availability rather than maximum reactivity. In general, primary and secondary amines are more reactive with acidic gases than tertiary amines. Similarly, primary amines are generally more reactive than secondary amines. Further, the configuration of the adsorbent may impose limitations on the nature of the amine that can be used.

CO₂ Adsorption Method and System

The present application further provides a method and a system for removing CO₂ exclusively or combined with other acid gases, such as H₂S, from a gaseous stream containing CO₂ and possibly one or more other acid gases. For simplicity, the following discussion specifically refers to CO₂ only, as the acid gas, however, it should be understood that other acid gases will also be removed, if they are available in the gaseous stream. Further provided is a system and process for CO₂ adsorption that include a means for, or the step of, contacting an amine-containing adsorbent alternatively with feed and purge gases at atmospheric or sub-atmospheric pressure, that have a relative humidity of at least about 0.1%, or as high as about 100%.

According to the presently described method and system, an amine-containing adsorbent is employed in a sorbent bed for use in a cyclic adsorption-desorption process carried out under humid conditions. In accordance with one embodiment, the CO₂ adsorption and desorption are carried out using feed and/or purge gases having a relative humidity of at least about 0.1%. In accordance with another embodiment, the feed and/or purge gases have a relative humidity of from about 0.1% to about 100%. In accordance with another embodiment, the purge gas may be a wet inert gas, a wet non-adsorbing gas, wet CO₂ or steam. In accordance with another embodiment, the total pressure of the purge gas is varying between 0.05 bar and 1 bar.

To apply an amine-containing adsorbent to such a cyclic adsorption-desorption process, the adsorbent must be formed into a stable, mechanically strong form. These forms may include, but are not limited to, powder forms, pellet forms, monolithic structures or foams. In the case of pellet forms, the adsorbent is mixed with a suitable inert or active secondary material as a binder. Criteria for selecting a suitable binder can include (i) achieving pellets or extrudates with minimum amount of binder; (ii) enhanced mechanical stability; (iii) preservation of adsorbent porosity and accessibility of adsorption sites; and (iv) affordability. For example, siloxanes and siloxane derivatives can be employed to form structured pellets, either extrudates or spheres, using the appropriate weight percentage of additive. The selection of the appropriate form and, if necessary, additive, is based on the application of the adsorbent and the type of equipment used in the CO₂ removal process. The selection and manufacture of the adsorbent form is well within the ordinary abilities of a worker skilled in the art.

Once the adsorbent form is selected and manufactured, it is used in a sorbent bed where a gaseous stream containing CO₂, and having a relative humidity of at least about 0.1%, contacts the adsorbent. The CO₂ and amine or CO₂, amine and water chemically react to form a complex, thereby removing the CO₂ from the gaseous stream.

According to a specific embodiment, once the adsorbent is loaded with CO₂ to a satisfactory level, for example, when greater than 80% of the amine has been converted to the amine complex, or at a designated cycle time, the sorbent bed can be regenerated. Regeneration comprises ceasing the flow of the gaseous stream through the bed and desorbing the adsorbed CO₂ at atmospheric or sub-atmospheric pressure. The endothermic desorption reaction is accomplished by thermal and/or pressure gradient means and by the use of a purge gas, or any combination thereof. During this step, the amine complex is dissociated, CO₂ is removed and the amine is freed and ready for re-use.

Various means can be incorporated in the system of the present invention to produce a relative humidity of at least about 0.1% in the feed and purge gases. In accordance with one embodiment, the system includes a water humidifier through which the feed and the purge gases used for adsorption and desorption are contacted with water at an appropriate temperature to achieve the desired relative humidity at the desired adsorption or desorption temperature. Other means by which the humidity of gases can be adjusted include, but are not limited to, heat and moisture exchangers and ultrasonic nebulizers.

It is understood that a suitable adsorbent is not necessarily limited to use for the exclusive removal of CO₂ from a gaseous stream. Rather the adsorbent can be used for the removal of additional acid gases, if they occur in the CO₂-containing gaseous feed.

In one embodiment, use of the adsorbent to remove CO₂, with or without other acid gases, can comprise utilising two or more sorbent beds operating cyclically such that the first bed is in the adsorption cycle while the second bed is in the desorption cycle. Such a system comprises two or more sorbent beds and computer or manually controlled valves and pumps allowing for continuous removal of CO₂ (and possibly other acid gases) from the gaseous stream. In the adsorption cycle, an exothermic reaction occurs between CO₂ in the gaseous stream, which is flowing through the adsorbent, and the amine present in the adsorbent, thereby adsorbing the CO₂ and forming a CO₂-amine complex. This complex can also be a combination of amine, CO₂ and water. In one embodiment of the present invention, the heat produced during the adsorption process in the first bed can be transferred via a heat exchanger to the second bed to drive the endothermic desorption of the adsorbed CO₂ and water simultaneously occurring therein. Alternatively, the desorption process can be effected through thermal and/or pressure gradient means independent of the adsorption process. Highly stable adsorbent is obtained when the gas feed used for adsorption and/or the purge gas used for desorption contain water vapour at a relative humidity of at least ca. 0.1% up to ca. 100%.

Among the different possibilities, moisture-containing CO₂ or steam can be used as purge gas to desorb CO₂. In both cases, pure CO₂ may be separated, and if need be, pressurized and stored.

To gain a better understanding of the invention described herein, the following examples are set forth. It should be understood that these examples are for illustrative purposes only. Therefore, they should not limit the scope of this invention in any way.

EXAMPLES Example 1 Adsorption-Desorption Cycling of Amine-Containing CO₂ Adsorbents in the Presence and Absence of Moisture

The loss of stability of amine-containing adsorbents through extensive CO₂ adsorption-desorption cycling under dry conditions, and the stabilization of amine-containing adsorbents through operation under humid conditions were investigated using triamine (TRI-PE-MCM-41) and, monamine (MONO-PE-MCM-41)-grafted pore-expanded MCM-41 mesoporous silica and polyethyleneimine-impregnated PE-MCM-41 (PEI-PE-MCM-41).

Materials

1. Preparation of Amine-modified Materials

TRI-PE-MCM-41 and MONO-PE-MCM-41 were prepared following the procedures described elsewhere (Sayari and Harlick 2010 and Serna-Guerrero et al. 2008, respectively. Briefly, MCM-41 type silica was produced at 100° C. in the presence of cetyltrimethylammonium bromide in basic conditions using tetramethylammonium hydroxide. Subsequently, the pore size of MCM-41 was increased by hydrothermal restructuring at 120° C. in the presence of an aqueous suspension of dimethyldecylamine [Sayari et al., 1998]. The surfactant template and pore expander agent were removed by calcination in nitrogen, then in air at 550° C., providing PE-MCM-41. The amine functional groups were incorporated onto PE-MCM-41 by grafting in toluene at 85° C. in a 250 mL glass reactor. First, 0.3 ml of water per g of silica was added to a PE-MCM-41 silica suspension in toluene, followed by addition of monoamine or triamine-containing silane (3 ml per g of silica). Grafting proceeded for 16 h, and then the product was filtered and washed with toluene and pentane.

Polyethyleneimine-containing material was prepared by dispersing PEI (molecular weight=324) on PE-MCM-41 as follows. An amount of 1 g of PEI was dispersed in methanol at room temperature followed by the addition of 1 g PE-MCM-41 to the mixture under continuous stirring. The mixture was stirred for 30 min and the slurry was dried overnight under ambient air.

Two amine-containing MOFs, namely amino-MIL-53 and Zn-aminotriazolato-oxalate were also prepared according to the methods set out in Ahnfeldt et al. 2009 and Vaidhyanathan et al. 2009, respectively.

Table 1 provides the structural properties and amine content of the materials, referred to as MONO, TRI and PEI.

TABLE 1 Characteristics of amine-containing CO₂ adsorbents Amine- Pore containing Surface area volume Pore size Amine Amine material S_(BET) (m²/g) V_(p) (cm³/g) D_(p) (nm) loading type^(a) MONO 420 0.65 7.2 4.3 mmol/g I TRI 367 0.87 9.4 7.9 mmol/g I, II PEI 4.5 * * 50 wt %   I, II, III ^(a)I: Primary; II: Secondary; III Tertiary. 2. Preparation of Mesoporous Silica Containing Urea Groups

The urea-containing species was incorporated onto PE-MCM-41 by grafting (bis(trimethoxysilylpropyl)urea in toluene at 85° C. using a 250 ml glass reactor. First, 0.3 ml of water per gram of silica was added to a PE-MCM-41 silica suspension in toluene, followed by addition of the silane (1.5 ml per gram of silica). Grafting proceeded for 16 h, and then the product was filtered and washed with toluene and pentane. The obtained material was labelled as UREA. The characteristics of the UREA sample were as follows: Urea loading: 4.3 mmol/g; surface area: 546 m²/g; pore volume: 0.8 cm³/g and pore diameter: 6.1 nm.

Cyclic CO₂ Adsorption-Desorption Studies

The cyclic CO₂ adsorption-desorption experiments were carried out using a Rubotherm magnetic microbalance (Belmabkhout and Sayari, 2009). The procedure was as follows: the sample was first activated in the presence of flowing UHP nitrogen at 50 mL/min for 30 min at a specified temperature (see below) under atmospheric pressure. Subsequently, the sample was cooled down to the adsorption temperature under isobaric condition and the feed gas was switched to pure CO₂ at 50 mL/min. The effective adsorption capacity (non-equilibrium) was assumed to be the weight gain of the sample after 30 min exposure. This procedure was repeated over tens or hundreds cycles (i.e., regeneration and CO₂ adsorption).

All the CO₂ adsorption-desorption cycling experiments reported herein were carried out at atmospheric pressure. The gas feed (pure CO₂) and the purge gas (pure N₂) used for desorption were either dry or contained a level of moisture corresponding to the equilibrium vapour pressure of water at 20° C. The actual relative humidity (RH) depended on the temperature of the gas stream, varying from 100% at 20° C. to 0.4% at 150° C. Under such conditions, the triamine-grafted material undergoing adsorption at X° C. and desorption at Y° C. under dry conditions will be referred to as TRI-X/Y-d. When the cycling experiment is carried out using moisture-containing gas streams, the material will be designated as TRI-X/Y-h. The same nomenclature applies to the other adsorbents.

FIG. 1 shows the dynamic CO₂ adsorption uptake over a number of adsorption-desorption cycles carried out at different conditions, in the presence of different adsorbents. As seen, in all cases, the materials deactivate at a rate that is dependent on the severity of the cycling conditions and on the nature of the adsorbent. The percentage loss of capacity was 14% for TRI-50/120-d over 40 cycles and as high as 41% for PEI-105/105-d for only 22 cycles. Under the same cycling conditions, adsorbents with grafted amines, MONO-105/105-d and TRI-105/105-d, deactivated by 45% over 40 cycles.

FIG. 2 shows the CO₂ adsorption capacity in the same series of experiments as in FIG. 1, except that the feed and the purge gases used for adsorption and desorption were bubbled through a water saturator maintained at 20° C. As seen, dramatic improvement in the stability of the materials was obtained through the use of humid conditions. In all cases, the adsorption capacity was stable throughout all cycles. Thus, this data demonstrates the enhanced stability of amine-containing CO₂ adsorbent in the presence of moisture-containing gases, in comparison to the stability in the presence of the same gases without added moisture.

¹³C CP MAS NMR data for a series of MONO samples are shown in FIG. 3. In addition to signals associated with the propyl carbon chain below 50 ppm, the fresh sample exhibited a signal at 165 ppm attributable to carbamate formed via adsorption of atmospheric CO₂. The MONO-105/105-d, which lost ca. 45% of its adsorption capacity upon CO₂ adsorption-desorption cycling under dry conditions (FIG. 1), exhibited an additional peak at 160 ppm. Interestingly, this peak did not occur in MONO-105/105-h, which underwent the same CO₂ cycling procedure, but in humid conditions, i.e., without loss of adsorption capacity. Thus, without wishing to be bound by theory, it was inferred that such species is responsible for the gradual loss of CO₂ adsorption capacity. To unambiguously identify this species, it was compared to the signal of the C═O group of bis(trimethoxysilylpropyl)urea-grafted MCM-41, referred to as UREA. Based on the data shown in FIG. 3, it is clear that the gradual deactivation of the adsorbents is due to the progressive formation of urea groups.

The contention that the formation of stable urea groups, favoured under dry conditions, is responsible for the deactivation of the amine-containing CO₂ adsorbents was further substantiated by in situ diffuse reflectance infrared Fourier transform (DRIFT). As shown in FIG. 4, as the number of CO₂ adsorption-desorption cycles increased, two bands at 1658 and 1560 cm⁻¹ developed. Comparison with the DRIFT spectrum of the UREA sample shows clearly that the two new bands are associated with the occurrence of urea groups.

The occurrence of urea groups takes place even under mild conditions, albeit at much slower rate. FIG. 5 shows that under mild, but dry conditions, TRI-70/70-d which seems to be stable over dozens of cycles, actually deactivates gradually over hundreds of cycles to reach 15% loss after ca. 750 cycles. As in the experiments described earlier, humidification of the gas streams led to a stable material over 700 cycles.

Scheme 1A shows the relationship between amine, CO₂, carbamate and urea species during CO₂ adsorption-desorption under dry conditions. The urea groups may form by direct interaction between amine species and CO₂ at high temperature. Drage et al., 2008 carried out temperature programmed adsorption of CO₂ on polyethyleneimine loaded silica under flowing CO₂. As the temperature increased, the weight decreased due to the carbamate decomposition. Then, starting at ca. 135° C., a weight increase corresponding to a secondary reaction between CO₂ and amine groups was associated with the formation of urea at high temperature. The experiments described herein demonstrate that under repeated CO₂ adsorption-desorption cycling, provided that the gas streams are dry, urea groups will form incrementally during adsorption and/or desorption (via carbamate decomposition) even under much milder conditions. Accumulation of heat-resistant urea groups leads ultimately to material deactivation. In contrast, Scheme 1B shows that if moisture-containing gases are used, the formation of urea is strongly inhibited. Indeed, even at a relative humidity as low as 0.4%, as in the case of TRI-70/150-h (FIG. 2), the material did not deactivate.

Deactivation via formation of urea groups can be reversed via hydrolysis of such species, while preserving the integrity of the material. Heating deactivated MONO-105/105-d at 200° C. under a flow of nitrogen containing as little as 0.15% RH (dew point at 20° C.) for 24 h restored the grafted propylamine completely, as evidenced by ¹³C NMR (FIG. 6) and CO₂ adsorption measurements. Nitrogen adsorption measurements at 77 K showed that the structural characteristics (Table 1) of the material were also preserved. Thus, the present invention further provides a method of recovering the adsorption properties for deactivated amine-containing CO₂ adsorbents. The method comprises the step of hydrolyzing urea groups formed during the adsorption phase.

Example 2 Preparation of Amine-Containing CO₂ Adsorbents Via Hydrolysis of Bis(Trialkoxysilylalkyl)Urea-Grafted Materials

As described above, the UREA sample was hydrolyzed under similar conditions (flowing N₂, 200° C., 0.15% RH, 24 h) to generate a propylamine-grafted PE-MCM-41, This is clearly shown by ¹³C NMR (FIG. 7), and DRIFT (FIG. 8). The material thus obtained proved to be an excellent CO₂ adsorbent with an equilibrium capacity of 1.3 mmol/g (5.7 wt %) at room temperature and 1 atm in the presence of 5% CO₂ in N₂. This corresponds to a CO₂ adsorption efficiency of CO₂/N=0.49.

REFERENCES

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All publications, patents and patent applications mentioned in this Specification are indicative of the level of skill of those skilled in the art to which this invention pertains and are herein incorporated by reference to the same extent as if each individual publication, patent, or patent applications was specifically and individually indicated to be incorporated by reference.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. 

The invention claimed is:
 1. A method of stabilizing an amine-containing CO₂ adsorbent over multiple adsorption-desorption cycles comprising: (a) an adsorption step of contacting said amine-containing CO₂ adsorbent with one or more humid gas streams to stabilize the adsorbent and to form a CO₂-loaded amine-containing adsorbent, and (b) a subsequent desorption step of contacting said CO₂-loaded amine-containing adsorbent with one or more humid or one or more dry gas streams, wherein said contacting steps are repeated during said multiple adsorption-desorption cycles.
 2. The method according to claim 1, wherein said one or more humid gas streams in said adsorption step is a humid CO₂-containing feed gas stream, or a humidified CO₂-containing feed gas stream.
 3. The method according to claim 1, wherein said one or more humid or one or more dry gas streams in said desorption step is a humid purge gas stream, a humidified purge gas stream or a dry purge gas stream.
 4. The method according to claim 1, wherein said amine-containing CO₂ adsorbent is within a system for removal of CO₂ from a humid or humidified CO₂-containing feed gas stream, and wherein: said adsorption step comprises contacting said amine-containing CO₂ adsorbent with a humid CO₂-containing feed gas stream, or a humidified CO₂-containing feed gas stream, and said desorption step comprises contacting said CO₂-loaded amine-containing adsorbent with a humid purge gas stream, a humidified purge gas stream or a dry purge gas stream.
 5. The method according to claim 4, wherein the purge gas stream is a humid, a humidified, or a dry inert gas; a humid, a humidified, or a dry non-adsorbing gas; a humid, a humidified, or a dry CO₂-rich gas; or steam.
 6. The method according to claim 1, wherein the relative humidity of humid gas streams is from about 0.1% to about 100%.
 7. The method according to claim 1, wherein said amine-containing CO₂ adsorbent is comprised, but not limited to, amines or polyamines grafted on inorganic matrices, organic matrices, hybrid matrices, organometallic matrices or a combination thereof, amines or polyamines impregnated on inorganic matrices, organic matrices, hybrid matrices, organometallic matrices or a combination thereof.
 8. The method according to claim 1, wherein said amine-containing CO₂ adsorbent is comprised of amines or polyamines grafted onto pore-expanded mesoporous silica. 